The interaction of calmodulin with Ca2+ and its target proteins has been studied in order to understand the mechanism of the regulation of cellular processes by Ca2+. Using large calmodulin fragments obtained in highly purified states by HPLC, we have identified the two high affinity Ca2+ binding sites as sites III and IV. Calmodulin fragment 78-148 (sites III and IV) interacts with two different enzymes and with anticalmodulin drugs. The NH2 terminus fragment 1-77 also interacts with anticalmodulin drugs required for activation of one enzyme studied but not the other. Thus, calmodulin contains at least two drug interacting domains and different domains are required for activation of different enzymes. Calmoculin exerts a tight coupling of Ca2 and cAMP regulation of cellular processes by controlling cAMP levels and cAMP dependent phosphorylation. It interacts with the regulatory subunit of cAMP dependent protein kinase and activates a Ca2+ regulated protein phosphatase, calcineurin. The amino acid sequence of the Ca2+ binding subunit of this enzyme has been determined in collaboration with Dr. P. Cohen.